Entropy/Darkness, his cards are potentially the evilest, first (s)he might screw up your quanta to give you a slow start, then Paradox every attacker, and what does survive gets blocked by a dissapation shield
Either that or you end up with 2 towers, while (s)he puts up 4 devourers and make you quantaless...
Usually slow and harmless, but rage quit potential.